The invention is concerned with the issue of how to produce n-pentanal by hydroformylation from feedstock mixtures comprising a small proportion of n-butene and a large proportion of n-butane. Specifically, solutions for further optimizing established processes for hydroformylation of such low-butene mixtures in terms of material utilization are sought.
The substance groups discussed in this connection are essentially alkenes (olefins), alkanes (paraffins), aldehydes and alcohols. These terms are used here in accordance with the terminology customary in chemistry.
In organic chemistry, substance groups are generally classified and named by the number of carbon atoms therein. The substance class of interest is preceded by the prefix Cn, where n is the number of respective carbon atoms present in the substance. When reference is made to C4 alkenes for example, this is understood to mean the four isomeric olefins having four carbon atoms, namely isobutene, 1-butene, cis-2-butene and trans-2-butene.
The saturated alkanes have barely any reactivity and are therefore used predominantly as fuel or aerosol propellant.
Meanwhile, it is possible to use the more reactive alkenes to form hydrocarbons having a greater number of carbon atoms which open up a broad spectrum of application and hence achieve higher sale prices than the starting materials having a smaller number of carbon atoms. This is how industrial organic chemistry adds value.
An economically important substance class which is produced from lower alkenes for this reason is that of the aldehydes. The aldehyde having three carbon atoms is called propanal. Two C4-aldehydes exist, namely n-butanal and isobutanal.
The aldehydes having five carbon atoms include the isomeric substances n-pentanal (also known as valeraldehyde), isopentanal (isovaleraldehyde), (S)-2-methylbutyraldehyde, (R)-2-methylbutyraldehyde and tert-pentanal. Valeraldehyde, used as a vulcanization accelerator, is of economic importance. Valeraldehyde may also be converted by aldol condensation and subsequent hydrogenation into 2-propylheptanol, an alcohol which is in turn a starting material for further syntheses toward PVC plasticizers, detergents and lubricants. Details may be found in U.S. Pat. No. 8,581,008.
n-Pentanal is produced by hydroformylation of n-butene.
n-Butene is an umbrella term for the three linear C4-olefins 1-butene, cis-2-butene and trans-2-butene. A mixture comprising these three isomeric substances is normally at issue; the precise composition depends on the thermodynamic state.
Hydroformylation (the oxo process) is generally understood to mean the conversion of unsaturated compounds such as in particular olefins with synthesis gas (hydrogen and carbon monoxide) into aldehydes having a number of carbon atoms one higher than the number of carbon atoms in the starting compounds. C5 aldehydes are accordingly produced by hydroformylating butene.
A comprehensive account of the current state of the art of hydroformylation may be found in:                Armin Börner, Robert Franke: Hydroformylation. Fundamentals, Processes and Applications in Organic Synthesis. Volumes 1 and 2. Wiley-VCH, Weinheim, Germany 2016.        
An established process for producing n-pentanal is disclosed in U.S. Pat. No. 9,272,973. The inventors proceed from this closest prior art.
In the hydroformylation for producing valeraldehyde practiced there an input mixture containing 35% 2-butenes and only 1% 1-butene is used. The remainder is butane which is inert toward the hydroformylation. The mixture extremely low in 1-butene is hydroformylated in the presence of a homogeneous catalyst system comprising a particular symmetrical biphosphite ligand which is stabilized by addition of an amine. Isononyl benzoate is mentioned as a solvent. With this catalyst system, butene conversions of 60% to 75% are achieved.
To enhance material efficiency WO2015/086634A1 proposes removing the unconverted alkenes from the reaction mixture using a membrane and converting them in a second hydroformylation stage with the aid of SILP technology. The inert alkanes are likewise discharged from the process with the membrane and thus do not cause any further disruption in the hydroformylation. This measure allows for very good material utilization, i.e. conversion into aldehydes, of the butenes present in the feedstock mixture. However, the butanes present in the feedstock mixture remain unutilized.
One option for better chemical utilization of alkanes than incinerating them is to dehydrogenate them. Dehydrogenation allows alkanes to be converted into more reactive and thus chemically versatile alkenes. Naturally, this requires energy. Since alkanes are relatively cheap raw materials a cost-effective dehydrogenation, especially performed in energy-efficient fashion, achieves significant added value. There is therefore a considerable range of commercially available technologies for dehydrogenation of alkanes, more particularly the C3-alkane propane, on offer. A comprehensive market analysis may be found in:                Victor Wan, Marianna Asaro: Propane Dehydrogenation Process Technologies, October 2015. Obtainable from IHS CHEMICAL, Process Economics Program RP267A.        
Since propane dehydrogenation is operated in the sphere of naphtha crackers these processes are all configured and optimized for a petrochemical-scale throughput. Thus the capacity of a propane dehydrogenation according to the UHDE STAR® process is approximately 500 000 t/a of propylene (see PEP-Report cited above, pages 2-11). These are scales which differ very markedly from those of industrially operated hydroformylation; thus the capacity of a large oxo plant is only 100 000 t/a (Börner/Franke, Introduction). It thus hardly makes economic sense to dehydrogenate propane with the aid of a costly large-scale plant and then to hydroformylate a small portion of the recovered propene unless the excess propene is used for instance for production of polypropylene.
It is apparently for this reason that literature reports of a combination of a dehydrogenation with a subsequent hydroformylation are conspicuous in their rarity:
In the field of C3-chemistry, U.S. Pat. No. 6,914,162B2 describes a combination of a propane dehydrogenation with subsequent hydroformylation of the obtained propene. The dehydrogenation is arranged before the hydroformylation in the upstream direction. In this connection “upstream” means further up the added-value chain. A similar process which also operates with n-butane as the feedstock is outlined in US2006/0122436A1.
WO2015/132068A1 likewise describes the dehydrogenation of C3 to C5 alkanes with downstream (i.e. in the direction of added value) hydroformylation. However, the latter is carried out in the presence of a heterogeneous catalyst system which is why this process differs markedly from the presently industrially operated, homogeneously catalyzed oxo processes in terms of apparatus.
Another reason why the dehydrogenation and the hydroformylation are not combined in practice is that the dehydrogenation affords not only the desired alkenes but also very many other hydrocarbons which are a great hindrance in the hydroformylation. One example thereof is 1,3-butadiene for instance which acts as an inhibitor in the hydroformylation. Such contaminants must first be removed from the alkenes at great inconvenience (i.e. cost) before said alkenes may be hydroformylated.
The intermediate removal of substances formed in the dehydrogenation and undesired in the hydroformylation is addressed in U.S. Pat. No. 8,889,935. However this process is used primarily to produce the linear C4-olefin 1-butene by dehydrogenation of n-butane. It is proposed in this connection that the 2-butene generated as a byproduct in the dehydrogenation of n-butane be converted into n-pentanal by hydroformylation. Contaminants formed in the dehydrogenation such as 1,3-butadiene are derivatized/selectively hydrogenated before hydroformylation.
U.S. Pat. No. 5,998,685 discloses a process where feedstock mixtures comprising n-butane and isobutane are dehydrogenated and the thus obtained alkenes are initially oligomerized and the obtained olefin oligomers are finally hydroformylated. Since the catalysts employed in the oligomerization are likewise very sensitive to byproducts generated in the butane dehydrogenation a costly and complex purification is interposed.
In terms of the prior art it can be said that due to the differences in throughput rates and the inevitably formed contaminants the alkane dehydrogenation with subsequent hydroformylation has acquired no practical significance.
However, the reverse combination, where the hydroformylation is arranged in front of the dehydrogenation in the upstream direction, is hardly found in the patent literature either:
Thus MY140652A discloses a process for producing oxo alcohols where the alkanes not converted in the hydroformylation are removed from the hydroformylation mixture and subjected to a dehydrogenation. The thus obtained alkenes are mixed with the fresh feedstock and also isomerized before hydroformylation. The feedstock originates from a Fischer-Tropsch process and essentially comprises alkanes and alkenes having 8 to 10 carbon atoms.
In this process the alkanes are removed by distillation from alkenes not converted in the hydroformylation at great cost and complexity before dehydrogenation. This is because the presence of alkenes in the dehydrogenation is undesirable since due to their up to four-fold higher reactivity compared to alkanes they form many oxidation products such as CO and CO2 which ultimately leads to rapid coking of the catalysts: cf. R. Nielsen: Process Economics Program Report 35F On-Purpose Butadiene Production II. December 2014, page 39 available from ihs.com/chemical. For the same reason providers of commercial dehydrogenation processes advise against introducing alkenes into the dehydrogenation.
The practical problem of contaminants requiring complex and costly removal is thus also present when a dehydrogenation is arranged downstream of a hydroformylation. In addition, even a world-scale oxo plant would scarcely be capable of keeping a dehydrogenation plant on a customary scale operating at anything approaching full capacity.
In conclusion it may be noted that the combination of dehydrogenation and hydroformylation or of hydroformylation and dehydrogenation is not industrially operated since the industrial scales do not coincide and because the specifications of the products and reactants of the respective processes are incompatible and thus necessitate a costly intermediate removal.